Many circuits require a certain amount of initial power to energize, prior to becoming self sustaining. For example, a flyback converter requires an initial amount of power for the control circuitry; however once the control circuitry is operative sufficient power is generated by the flyback converter to drive the control circuitry, such as by an auxiliary winding as part of a bootstrap circuit. Circuitry providing the initial power for the control circuitry is alternately known as a start up circuit or a kick start power source.
Prior art methods of starting up a switched mode power supply typically tap power after a rectifier, and use a voltage divider comprising resistive elements, or a resistor and capacitor combination to supply the start up power. Unfortunately, even after start up power is dissipated through the start up circuit resistive path. Other prior art methods include utilizing a high voltage constant current source, and after start up the constant current source is shut down. However, during start up there is high power dissipation in the start up circuit in the event of a high input voltage.
LED lighting circuits are rapidly coming into wide use, and are preferably designed for use with a wide range of AC voltages found around the world. A single circuit, designed for use with a broad range of voltages, enables reduced cost, since there is no need to have changes between devices produced for various markets. It is to be noted, that the range of AC voltages experienced worldwide, over a broad range of conditions, typically varies from 90 VAC to 350 VAC, RMS.
What is desired, and not supplied by the prior art, is a start up power circuit that does not appreciably dissipate power, and that is preferably useable with a broad range of input voltages.